An exhaust-gas system for diesel combustion engines generally consists of the following components:                DOC for oxidizing hydrocarbons and as cold-start hydrocarbon reservoir;        DPF for reducing particle emissions;        optionally an H catalyst for urea treatment;        SCR catalyst for reducing nitrogen oxides;        barrier catalyst as ammonia oxidation catalyst.        
By DOC (diesel oxidation catalyst) a person skilled in the art understands a catalyst which on the one hand acts as a cold-start hydrocarbon reservoir and in normal operation oxidizes unburnt hydrocarbons. The treatment of exhaust gases of diesel combustion engines with catalysts requires changes to the design of catalyst materials as, unlike a petrol engine, a diesel engine is always run with an excess of oxygen and the catalyst is thus never subjected to reductive conditions.
Particle filters (DPF, diesel particle filters) are used to filter out soot particles from the exhaust gas of combustion engines, specifically diesel engines, and thus to reduce their discharge into the atmosphere. Various filter designs, such as e.g. so-called “wall-flow filters” or filters made from ceramic or metal foams, are used. However, the real problem is not the filtration of the soot particles, but the regeneration of the filters used. Depending on the operation-governed composition of the particles, carbon black spontaneously combusts only at temperatures between 500° C. and 700° C. However, modern diesel engines e.g. generally reach these temperatures only at full load.
Therefore, additional supporting measures, for example for oxidizing the soot particles separated from the exhaust gas, are necessary. This can occur by adding additives or by a catalytic coating of the filters or catalysts. Exhaust-gas cleaning catalysts which have a high oxidation action, so that the particles can be combusted at a low temperature, are known from the state of the art. The surface of the filter chamber therefore often has a catalytically active coating to accelerate the combustion of the soot particles collected on the filter. The catalytically active coating oxidizes the nitrogen monoxide contained in the exhaust gas to nitrogen dioxide. The nitrogen dioxide formed then improves the oxidation of the deposited particles.
SCR (“selective catalytic reduction”) denotes the selective catalytic reduction of nitrogen oxides from exhaust gases of combustion engines (and also power stations). Only the nitrogen oxides NO and NO2 are selectively reduced with an SCR catalyst, wherein NH3 (ammonia) is usually admixed for the reaction. Only the harmless substances water and nitrogen form as reaction product. The addition of ammonia as well in compressed-gas bottles is a safety risk for use in motor vehicles. Therefore precursor compounds of ammonia which are broken down in the exhaust-gas system of the vehicles accompanied by the formation of ammonia are customarily used. For example the use of AdBlue®, which is a 32.5% eutectic solution of urea in water, is known in this connection. Other ammonia sources are for example ammonium carbamate, ammonium formate or urea pellets. A hydrolysis catalyst (H cat) which generates NH3 from the precursor substances is therefore often also used.
Ammonia must first be formed from urea before the actual SCR reaction. This occurs in two reaction steps which together are called hydrolysis reaction. Firstly, NH3 and isocyanic acid are formed in a thermolysis reaction. Isocyanic acid is then reacted with water in the actual hydrolysis reaction to ammonia and carbon dioxide.
To avoid solid depositions it is necessary for the second reaction to take place sufficiently quickly by choosing suitable catalysts and sufficiently high temperatures (from 250° C.). Simultaneously, modern SCR reactors act as the hydrolysis catalyst.
The ammonia formed through thermohydrolysis reacts at the SCR catalyst according to the following equations:4NO+4NH3+O2→4N2+6H2O  (1)NO+NO2+2NH3→2N2+3H2O  (2)6NO2+8NH3→7N2+12H2O  (3)
At low temperatures (<300° C.) the conversion proceeds predominantly via reaction (2). For a good low-temperature conversion it is therefore necessary to set a NO2:NO ratio of approximately 1:1. Under these conditions the reaction (2) can already take place at temperatures from 170-200° C.
The oxidation of NO to NOx takes place in an upstream oxidation catalyst which is necessary for an optimum degree of efficiency.
If more reductant is added than is converted during the reduction with NOx, an undesired NH3 slip may result. The NH3 is removed in the state of the art by an additional oxidation catalyst behind the SCR catalyst. This barrier catalyst oxidizes any ammonia that may occur to N2 and H2O. It is also essential that the urea dose be applied carefully.
An important characterizing variable for SCR catalysis is the so-called feed ratio α, defined as the molar ratio of added NH3 to the NOx present in the exhaust gas. Under ideal operating conditions (no NH3 slip, no secondary reactions, no NH3 oxidation), α is directly proportional to the NOx reduction rate:
With α=1 a one hundred per cent NOx reduction is theoretically achieved. In practical use a NOx reduction of 90% can be achieved in stationary and non-stationary operation with an NH3 slip of <20 ppm.
With today's SCR catalysts a NOx conversion >50% is achieved by the upstream hydrolysis reaction only at temperatures of above approx. 250° C., optimum conversion rates are achieved in a temperature range of from 250-450° C.
The dosing strategy is very important in catalysts with large NH3 storage capacity, as the NH3 storage capacity of SCR catalysts of the state of the art typically falls as the temperature rises.
At present SCR catalysts based on titanium dioxide, vanadium pentaoxide and tungsten oxide are predominantly used both in the field of power stations and in the automobile field. The use of SCR catalysts based on zeolites is also known in the state of the art. However, in this case the zeolite acts only as SCR active component.
According to state of the art, a downstream ammonia barrier catalyst which oxidizes excess ammonia from the SCR catalyst is frequently also used, as ammonia is very harmful to health and the environment.
As can be seen, a modern exhaust-gas system comprises a large number of components which are usually integrated into the exhaust-gas stream on the base of the vehicle. As the space available there is limited, it would thus be desirable if the available space could be used more effectively.
Therefore, efforts have already been made in the state of the art to combine several catalyst functions in one component.
WO 2005/016497 A1 discloses to this end a combination of a particle filter with an SCR active component.
A filter substrate which is coated with a NOx storage component on the exhaust-gas entry side and with an SCR active component on the exhaust-gas exit side is disclosed in DE 10 335 785 A1.
WO 01/12320 A1 discloses a catalytic wall-flow filter which is coated with an oxidation catalyst for HC, CO and NO on the exhaust-gas entry side and with a NOx absorber and a NOx reduction catalyst on the exhaust-gas exit side. The filter substrate lying between them does not have a catalytic coating and serves only to filter particles. The same principle is disclosed in US 2004/0175315 A1. The principle of the CRT (continuous regeneration trap) is disclosed in both documents, wherein filter regeneration is not necessary.
With non-continuous systems in which a filter regeneration is periodically necessary, the formation of CO during the regeneration phase represents a problem. Soot is oxidized not only to CO2 but partly also to CO. However, CO is an undesired component in exhaust gas.
A further problem of particle filters provided with SCR active components is that very high temperatures (up to 800° C.) occur during the regeneration phase of the filter. However, NH3 breaks down from a temperature of approximately 630° C., even earlier in the presence of metal oxides. For this reason, the urea dosing must be interrupted during filter regeneration, whereby there is no conversion of the nitrogen oxides during the regeneration phase.